Conventional multi-arm rotational moulding machines usually include a central rotary hub which is floor mounted and carries several spaced radially projecting cantilever arms which are rotatably located in large bearings in the hub. The free end of each arm carries a structure on which moulds are releasably mounted for manipulation on two axes. The hub, in use, rotates the arms and their mould manipulation structures sequentially through heating, cooling and stripping stations which are located on a circle about the hub axis. The hub is generally a very complex, robust and expensive item of machinery as it contains all of the motors, electric controls, couplings and mechanicals necessary to:    a) rotate each tubular arm in both directions.    b) rotate a coaxial drive shaft within each tubular arm in both directions. The coaxial drive shaft generally carries, at the manipulator, a relatively small diameter bevelled gear which drives a second bevelled gear to rotate the mould carrying structure about its second axis. Because of the small sizes of the gears and so the short torque arm relative to the size of the moulds being manipulated the gears are subjected to enormous torque loads, particularly in the case where the structure is supporting an unbalanced mould, with the result that enormous loads are also placed on the bearings supporting the bevelled gears. The overhung cantilever design of the radial arms additionally results in high cyclic stress reversals due to the nature of the bi-axial rotational movement of the mould mounting structure which may, and not infrequently does, subject the machine mechanisms to premature metal fatigue failure. The gears and their support bearings are additionally operated in extremely hostile environments which are caused by the cyclic heating and cooling of each manipulator as it is rotated through the static heating and cooling stations of the machine further to increase the high wear rates of the already severely loaded components of the mould carrying structure.    c) be sufficiently robust to withstand the lifting forces applied to the arm when stripping a complex mould vertically from the mould carrying structure.    d) provide services through slip-rings and rotary glands.A further problem with the machines of the prior art is the inefficient use of heat. The heating station ovens generally have two doors, which open to allow an arm carrying a mould manipulating structure to be rotated through the heating station. Alternatively the ovens in some cases are provided with sliding doors which face the machine hub and which when opened allow the entire oven housing to be retracted in a radial direction away from the arm moulds. Either way a large amount of heat in the oven is lost to atmosphere when the doors are opened. Additionally, the oven housings, due to their box-type shape have an internal volume far in excess of what is required with this drastically reducing the thermal efficiency of the oven. This loss of heat requires, during the subsequent heating cycle of the machine, that the heating chamber needs almost totally to be reheated to moulding temperature prior to the effective commencement of the moulding cycle in the oven housing. As the heating cycle of any rotational moulding machine determines the cycle time of the machine, this warm-up heating period during each heating cycle, expensively increases heating costs and the productive machine cycle time.
In addition to the above multi-arm moulding machines are shuttle-type machines such as those disclosed in the specifications of WO96/38281 and U.S. Pat. No. 4,690,626 in which the mould manipulators are moved in and out of a moulding oven through doors with a large degree of heat loss as occurs with the multi-arm machines.
The specification of U.S. Pat. No. 4,292,015 disclosures four moulding ovens which each include a rock and roll type manipulator and a gas burner. The machines are mounted on a horizontally rotatable platform. To cool the moulds, they are sequently removed from the ovens and are then quenched in a liquid bath which is remote from the ovens. The four dedicated gas burner make this moulding apparatus heat extravagent.
Yet another problem with the above machines of the prior art is that the machines generally require a large amount of floor space with this being aggravated by machines of the type which employ the radially moving heating oven housings.